The present invention relates generally to an improved process and equipment for converting feedstock into useful materials, and more specifically, to an anaerobic fermentative process for bioconverting animal waste, sewage sludge or other biodegradable feedstock into methane gas, carbon dioxide gas, ammonia, carbon black, organic acid, charcoal, a fertilizer and/or an insecticidal mixture.
Animal waste poses a significant problem in the poultry, swine and cattle industries. Animal waste from animal raising or processing operations is responsible for a significant amount of underground water contamination and methods are continually being developed for handling animal wastes. One known method is the bioconversion of animal waste into useful products.
Methods for the anaerobic digestion or treatment of sludge, animal waste, synthesis gas or cellulose-containing waste are disclosed in U.S. Pat. Nos. 5,906,931 to Nilsson et al., No. 5,863,434 to Masse et al., No. 5,821,111 to Grady et al. No. 5,746,919 to Dague et al., No. 5,709,796 to Fuqua et al., No. 5,626,755 to Keyser et al., No. 5,567,325 to Townsley et al., No. 5,525,229 to Shih, No. 5,464,766 to Bruno, No. 5,143,835 to Nakatsugawa et al., No. 4,735,724 to Chynoweth, No. 4,676,906 to Crawford et al., No. 4,529,513 to McLennan, No. 4,503,154 to Paton, No. 4,372,856 to Morrison, No. 4,157,958 to Chow, and No. 4,067,801 to Ishida et al. These patents disclose different processes and equipment for the bioconversion, either by microbial digestion or enzymatic conversion, of those materials into methane and other useful materials.
The equipment used for the anaerobic digestion or fermentation of waste into fuel, such as methane, varies greatly and is generally tailored to specific applications. Equipment that is suitable for a first type of feedstock generally has to be modified before it can be used for a second different type of feedstock.
Chemical and biochemical reactions that create a gas are generally conducted at low to sub-atmospheric pressures due to the tendency of the product gas to function as feedback inhibitor that inhibits further formation of the gas. The art recognizes that variations in the pressure of an anaerobic digester can be used to effect different biochemical and productivity results. U.S. Pat. No. 4,409,102 to Tanner discloses an anaerobic digestion conducted at sub-atmospheric pressures that unexpectedly affect an increase in methane gas production. U.S. Pat. No. 3,994,780 to Klass et al. discloses the high pressure rupture of cells in an anaerobic digester to render cellular components available to other intact cells in the digester. U.S. Pat. No. 3,981,800 to Ort discloses a process for preparing high quality methane (about 98% wt.) with an anaerobic digester operated at 1-5 atm. above atmospheric pressure provided that the sludge is degassed by a recirculator and passed between two digesters connected serially to remove carbon dioxide in the sludge that is then fed back into the digester. Therefore, unlike the presently claimed system, the system or Ort requires that each batch of feedstock under go a two-stage digestion, wherein the feedstock is predigested in a first anaerobic digester and then completely digested in a second anaerobic digester that is connected serially with the first anaerobic digester. U.S. Pat. No. 4,100,023 to McDonald discloses that the internal pressure of the anaerobic digester should be kept at about 1 to 3 inches of water column to ensure proper performance. U.S. Pat. No. 4,568,457 to Sullivan discloses a two-stage anaerobic digester system having an acid forming stage and a methane gas forming stage, wherein the pressure of the gas in the headspace of the two stages can be slightly above atmospheric pressure.
Methanogenic microbes that create methane from carbon and hydrogen containing feedstocks, such as cellulose, animal waste, food processing waste, and sludge, are well known. These microbes have been used in the waste processing industry and are available in their native forms from natural sources or in genetically altered or manipulated forms, which can produce greater amounts of useful materials per unit weight of waste than can unaltered methanogenic bacteria.
To date, no equipment containing the required components as described herein has been disclosed. Further, the improved equipment design and layout of the present invention provides a higher yield of methane and other useful materials than other comparable equipment. Still further, the improved process and equipment of the invention can be used in the poultry, swine, dairy or cattle industries to convert cellulose-containing animal waste into methane which is used to operate farm or ranch equipment thereby reducing operating costs and the volume of waste produced.
The present invention provides a system for converting cellulose-containing feedstock into useful materials, wherein the system comprises:
a feedstock slurry feeder;
a plurality of conduits connecting various components of the system;
a single pressurizable anaerobic digester comprising agitation means, one or more feed ports, one or more discharge ports, an optional pressure regulator, and a reaction vessel for holding a reaction solution comprising an anaerobic microbe which converts an aqueous slurry of cellulose-containing feedstock into at least methane and an enriched effluent;
a pressurizer; and
one or more gas processors directly or indirectly connected to the anaerobic digester;
wherein the headspace of the anaerobic digester is pressurized to about 10 psi or more to form the enriched effluent and a discharge gas comprising at least methane during anaerobic digestion of the feedstock slurry.
Depending on the feedstock slurry used, the anaerobic digester will also form a fertilizer, sludge, scum, ammonia, charcoal, carbon black, an organic acid and/or an insecticidal mixture. The anaerobic digester is preferably operated at pressures between 10 to 265 psi, more preferably 10 to 100 psi, and even more preferably 25-75 psi. In preferred embodiments, the system also comprises one or more of the following: one or more gas scrubbers, one or more heaters for heating or preheating the slurry being digested in the anaerobic digester, one or more water storage tanks, one or more feedstock slurry tanks, one or more feedstock grinders, one or more supernatant storage tanks, one or more sludge storage tanks, one or more sludge dryers, one or more scum storage tanks, one or more CO2 tanks, and/or one or more produced gas storage tanks.
Other preferred embodiments include those wherein the system does not require a water lagoon, a foam trap, and/or a water vapor trap. Still other preferred embodiments include those wherein: (1) the system is operated in a batch, semi-continuous, or continuous mode; (2) the feedstock slurry comprises from about 1-90% wt. solids, more preferably about 1-60% wt. solids, or even more preferably about 1-40% wt. solids; (3) the agitation means comprises a gas bubbler, an aerator, a sparger bar, a fluid stream, a mechanical agitator, or a combination thereof; (4) the feedstock slurry is gravity fed or fed under pressure to the anaerobic digester; (5) the pressurizer pressurizes the anaerobic digester with gas or a liquid; (6) the pressurizer is the feedstock slurry feeder, which is preferably a pump, gravity feed system, or a gas compressor; (7) the anaerobic digester does not require aerobic digestion of the feedstock; (8) the anaerobic digester does not require multiple discrete zones of environmentally incompatible waste-digestive microorganisms; (9) the anaerobic microbe is a methanogenic bacterium; (10) the anaerobic microbe is mesophilic or thermophilic; (11) methane produced by the anaerobic digester is used to operate an internal combustion engine, an electrical current generator, an electric engine, a water heater, a furnace, an air conditioning unit, a ventilation fan, a conveyor, a pump, a heat exchanger, fuel cell, or various components of the system itself and/or to recharge power cells; (12) the gas processor comprises a gas scrubber and/or a gas separator; (13) a gas recirculator is used to recirculate gas from the headspace of the reactor to the slurry in the reactor; (14) a gas recirculator adds methane-depleted or carbon dioxide enriched biogas back to the reactor; and/or (15) a fluid recirculator recycles the scum, supernatant, effluent, or sludge of the reactor.
Another aspect of the invention provides a system for converting cellulose-containing feedstock into useful materials, wherein the system comprises:
one or more feedstock slurry feeders;
two or more pressurizable anaerobic digesters connected in parallel, each anaerobic digester comprising agitation means, one or more feed ports, one or more discharge ports, an optional pressure regulator, and a reaction vessel for holding a reaction solution comprising an anaerobic microbe which converts an aqueous slurry of cellulose-containing feedstock into at least methane and an enriched effluent;
one or more pressurizers;
one or more gas processors directly or indirectly connected to each anaerobic digester; and
a plurality of conduits connecting various components of the system;
wherein the headspace of each anaerobic digester is pressurizable to about 10 psi or more to form the enriched effluent and a discharge gas comprising at least methane during anaerobic digestion of the feedstock slurry.
Specific embodiments of this aspect of the invention include those wherein: 1) a major portion of the discharge gas is methane; 2) the system comprises a single feedstock slurry feeder connected to each of two or more pressurizable anaerobic digesters connected in parallel; 3) the enriched effluent from a first pressurizable anaerobic digester is not fed into a second pressurizable anaerobic digester; 4) the system further comprises one or more receiving tanks that receive the enriched effluent from each pressurizable anaerobic digester.
Another aspect of the invention provides an integrated system for converting cellulose-containing feedstock into useful materials, wherein the integrated system comprises:
a feedstock slurry feeder system that forms an aqueous slurry of cellulose-containing feedstock;
an anaerobic digester system directly or indirectly connected to the feeder system and comprising two or more pressurizable anaerobic digesters connected in parallel, wherein each anaerobic digester receives the aqueous slurry of cellulose-containing feedstock and converts it into a discharge gas and an enriched effluent; and
a discharge gas processing system that is directly or indirectly connected to the anaerobic digester system and that at least separates methane from the discharge gas;
wherein the headspace of each anaerobic digester is pressurizable to about 10 psi or more to form the enriched effluent and a discharge gas comprising at least methane during anaerobic digestion of the feedstock slurry.
Specific embodiments include those wherein: 1) the integrated system further comprises an enriched effluent processing system; 2) the integrated system further comprises a pressurizer system; 3) the feedstock slurry feeder system comprises one or more mixing vessels and one or more pumps; 4) the discharge gas processing system comprises a dehydrator, separator, and scrubber; 5) each anaerobic digester comprises agitation means, one or more feed ports, one or more discharge ports, an optional pressure regulator, and a reaction vessel for holding a reaction solution comprising an anaerobic microbe that converts the aqueous slurry of cellulose-containing feedstock into at least methane and the enriched effluent; 6) the discharge gas processing system comprises one or more separators for separating CO2 or ammonia gas from the discharge gas; 7) the integrated system further comprises a gas recirculator to recirculate gas from the headspace of an anaerobic digester to the slurry of the digester; 8) a gas recirculator adds methane-depleted or carbon dioxide enriched discharge gas back to an anaerobic digester; and/or 9) the integrated system further comprises a fluid recirculator system that recycles the scum, supernatant, effluent, or sludge of an anaerobic digester.
Another aspect of the invention provides an integrated anaerobic digester system comprising:
a single-stage anaerobic digester system comprising two or more pressurizable anaerobic digesters connected in parallel, wherein each anaerobic digester receives an aqueous slurry of cellulose-containing feedstock and converts it into a discharge gas and an enriched effluent;
a discharge gas processing system that is directly or indirectly connected to the anaerobic digester system and that at least separates methane from the discharge gas; and
an enriched effluent processing system that is directly or indirectly connected to the anaerobic digester system;
wherein the headspace of each anaerobic digester is pressurizable to about 10 psi or more to form the enriched effluent and a discharge gas comprising at least methane during anaerobic digestion of the aqueous slurry of cellulose-containing feedstock.
Other features, advantages and embodiments of the invention will be apparent to those skilled in the art by the following description, accompanying examples and appended claims.